The typical double-ended incandescent lamp comprises a tubular light-transmitting envelope having a central longitudinal axis, conductive terminals at opposite ends of the envelope, and a light-generating filament located within the envelope and connected between the terminals. The terminals of such lamp typically have concave surfaces that face axially outward with respect to the envelope. The luminaire that receives the lamp includes a housing and a reflector within the housing for aiming the light generated by the lamp when the lamp is installed. A pair of spaced-apart contacts are located within the housing and have convex portions adapted to respectively engage the concave surfaces of the lamp terminals when the lamp is installed within the luminaire. Each terminal of the lamp is provided with a sleeve of electrical insulation that covers all exterior surfaces of the terminal except for the outwardly-facing concave surface, which is slightly recessed from the outer end of the sleeve. The presence of the insulating sleeves and the recessed character of the concave surfaces reduce the chances that a live part of the terminals will be accidentally touched during relamping, and this reduces the chances for electric shock to the installer.
A luminaire of the above type should be disconnected before any relamping is attempted, and there is usually a caution notice on the luminaire that warns against the risk of electric shock if the luminaire is not disconnected during relamping. In the event that the warning notice is not heeded and the luminaire remains connected during relamping, a situation may arise where both terminals of the lamp are live during the relamping process even though only one terminal is in engagement with its mating contact in the luminaire. The insulating sleeve around the still disengaged terminal will normally prevent contact with this disengaged terminal, but there is a possibility that the recessed conductive portion of the still-disengaged terminal will be touched during relamping, and this could result in serious electric shock.
It is possible to reduce the possibilities of such electric shock by providing an interlock switch that automatically disconnects power to the contacts when the luminaire is opened for relamping. But such switches are not fool-proof and, moreover, they can be quite expensive, considering that they must usually be designed to withstand the high temperatures developed by the luminaire.